no matter what I do, the hair net addon doesn't work (Blender 2.9)
HairNet version
For Blender 2.90.1 you need HairNet 0.6.3. Version 0.6.2 is for Blender 2.90.0 (and older) and you will get this &quot;mouse_event&quot; error when you use it in a later version of Blender.
Sheets
A common mistake with the sheets is to use a sheet with only 2 &quot;strand edges&quot;. The add-on needs at least 3 strand edges to work. Else you get this misleading &quot;no seams&quot; error.

Additionally

make sure that Location, Scale, and Rotation  is applied to the hair objects and to the head (Ctrl+A in Object mode).
make sure that you don't have accidentally checked the HairNet option [_] Emit Hair on Self when you have a head (sheet and head for example). Check both objects because the setting is not global but stored in the objects.
select the head/emitter object last (orange selection color)
the HairNet 0.6.3 (or Blender) has a bug the makes all hair of a random existing hair particles system move down on the z-axis if you add another hair particle system from sheets, curves, etc. You can prevent this if you turn off the previous particle systems in the viewport with the &quot;monitor&quot; icon before you add another one with the add-on.
don't forget to increase the number of Strand Steps (5) in the Viewport Display* settings for a better shape
and to &quot;touch&quot; the created particle guides in Particle Edit mode with a minimal strength brush as described in the manual